BS digital broadcasting and terrestrial digital broadcasting have been successively started since 2000, and thereby demand for digital television tuners is increasing year after year, and development thereof is actively carried out by makers and research institutes.
On the other hand, in recent years, thin-screen televisions such as plasma televisions and liquid crystal televisions are becoming a mainstream instead of conventional CRT-based television. Therefore, cost reduction and downsizing of various parts including tuners are strongly demanded.
However, in many cases, reception units of existing tuners are constituted by bipolar chips that are superior in reception sensitivity characteristics, i.e., noise characteristics, while digital signal processing units thereof are constituted by CMOS chips that are superior in cost reduction and downsizing. Accordingly, the tuner system is constituted by two chips, and it is becoming incapable of satisfying the demands of users for cost reduction and downsizing.
As a means for satisfying the user demands, it is known to realize the tuner system on one chip by constituting the reception unit with CMOS, which has been constituted by bipolar.
However, generally, the characteristics of a CMOS device are inferior to those of a bipolar device, and a sufficient performance cannot be obtained by only replacing the circuit as it is with CMOS. Particularly, the reception sensitivity characteristics of CMOS are disadvantageous relative to those of bipolar in many cases.
Accordingly, in the design of the CMOS tuner system, the design of the reception unit, particularly, the design of a low-noise amplifier circuit which almost determines the reception sensitivity characteristics is important, and the low-noise amplifier circuit is required to have a large gain to enhance the noise characteristics in the reception unit. In order to objectify this matter, a description will be given of the noise characteristics of a tuner system as shown in FIG. 29.
FIG. 29 is a block diagram illustrating a tuner system.
In FIG. 29, a direct conversion method is adopted as a reception method. Reference numeral 14 denotes an antenna, 15 denotes a low-noise amplifier circuit, 16a and 16b denote mixers, 17a and 17b denote LPFs (Low Pass Filters), 18a and 18b denote VGAs (Variable Gain Amplifiers), 19 denotes a 90° phase shifter, 20 denotes a PLL, and 21 denotes a digital circuit.
In order to calculate a noise factor F of the reception unit, i.e., from the antenna 14 to the VGA 18, it is assumed that the gains of the low-noise amplifier circuit 15, the mixers 16a and 16b, the LPFs 17a and 17b, and the VGAs 18a and 18b are G15, G16, G17, and G18, respectively, and the noise factors thereof are F15, F16, F17, and F18, respectively. At this time, it is known that the noise factor F is given from the Friis formula as follows.
                    F        =                              F            ⁢                                                  ⁢            15                    +                                                    F                ⁢                                                                  ⁢                16                            -              1                                      G              ⁢                                                          ⁢              15                                +                                                    F                ⁢                                                                  ⁢                17                            -              1                                      G              ⁢                                                          ⁢                              15                ·                G                            ⁢                                                          ⁢              16                                +                                                    F                ⁢                                                                  ⁢                18                            -              1                                      G              ⁢                                                          ⁢                              15                ·                G                            ⁢                                                          ⁢                              16                ·                G                            ⁢                                                          ⁢              17                                                          (        1        )            
With reference to formula (1), the gain G15 of the low-noise amplifier circuit 15 is included in all terms from the second term on the right side. Accordingly, the noise factor F strongly depends on G15, and the noise factor F can be reduced by increasing G15. As a result, the noise characteristics of the reception unit can be enhanced (e.g., refer to Non-patent Document 1).
On the other hand, in a reception system that handles a broadband signal, such as a digital television tuner, the low-noise amplifier circuit is required to have a broad input-signal bandwidth of 1 GHz or more.
FIG. 28 is an image diagram of a RF signal that is input to a BS/CS digital television tuner system.
For example, in the BS/CS digital television tuner system, as shown in FIG. 28, it is necessary to receive signals of about 20 channels which exist in a band from 0.95 GHz to 2.15 GHz. That is, it is necessary to realize equal reception sensitivity characteristics for all the channels, and the low-noise amplifier circuit is required to have flat gain characteristics in the frequency band.
However, generally, the gain and the bandwidth are in the trade-off relation, and it is difficult to realize both of them at high levels. Especially in the case of the CMOS, since the noise characteristics of the CMOS device are inferior to those of the bipolar device, the demand for high gain to the low-noise amplifier circuit becomes more severe.
In order to explain the above-described matter more specifically, the conventional low-noise amplifier circuit will be described hereinafter.
FIG. 24 is a circuit diagram illustrating a configuration of a conventional low-noise amplifier circuit 1100, FIGS. 25 and 26 are circuit diagrams illustrating conventional low-noise amplifier circuit 1200 and 1300 having other configurations, and FIG. 27 is a characteristic diagram illustrating trade-off between gain and band in the conventional low-noise amplifier circuit.
In the conventional low-noise amplifier circuit 1100 shown in FIG. 24, its fundamental construction is an inductor-loaded source-grounded amplifier circuit. Reference numeral 1 denotes a signal amplifier, 2 denotes a load unit, 3 denotes a RF signal supplier, 5 denotes an input terminal, 6 denotes an output terminal, 7 denotes a transistor, 10 denotes an inductor, 11 denotes a capacitor, and 13 denotes a broadbanding resistor. However, the RF signal supplier 3 is a general means for supplying a RF signal, and it is an antenna 14 in the tuner system shown in FIG. 29.
Hereinafter, the operation of the conventional low-noise amplifier circuit 1100 will be described with reference to FIG. 24.
An input signal supplied from the RF signal supplier 3 is input to a gate terminal of the transistor 7, and converted from a voltage signal to a current signal. Then, the current signal flows in the load unit 2 to be converted into a voltage signal again, and the voltage signal is outputted from the output terminal 6 as an output signal.
It is known that the frequency characteristics of the gain of the low-noise amplifier circuit are determined according to the characteristics of the load unit 2. To be specific, the gain becomes maximum with the resonance frequency that is determined by the inductance L10 of the inductor 10 and the capacitance C11 of the capacitor 11. Usually, the circuit designer controls L10 or C11 according to the application to match the resonance frequency to the frequency band of the input RF signal.
The broadbanding resistor 13 is connected to reduce the Q value (steepness of load impedance characteristics) of the load unit 2, and the gain characteristics can be broadbanded as shown in FIG. 27 by reducing the resistance R13 of the broadbanding resistor 13.
Further, although specific description will be omitted, a low-noise amplifier circuit 1200 including a load unit 2 in which a broadbanding resistor 13 is connected in series with an inductor 10 as shown in FIG. 25, and a low-noise amplifier circuit 1300 including a signal amplifier 1 in which a negative feedback resistor 22 and a DC block capacitor 23 are connected in series between an input terminal 5 and an output terminal 6 as shown in FIG. 26, are also able to broad-band the gain characteristics as shown in FIG. 27 like the low-noise amplifier circuit 1100 shown in FIG. 24 (e.g., refer to Non-patent Document 2).
Non-patent Document 1: “RF microelectronics”, written by Behzad Razavi, translated by Tadahiro Kuroda, Maruzen Co., Ltd., March 2002, p. 47-50
Non-patent Document 2: “The Design of CMOS Radio-Frequency Integrated Circuits”, written by Thomas H. Lee, CAMBRIDGE UNIVERSITY PRESS, 1998, p. 178-222